1. Field of the Invention
The present invention relates to an engine control system and, more particularly, to an ignition timing control system for an automotive internal combustion engine which suppresses shocks caused by the engine due to rapid changes in engine output torque.
2. Description of Related Art
Engines installed in automotive vehicles are conventionally designed to retard ignition timing in order to drop engine output torque when the engine operates so that its output torque changes rapidly. This is done because when the engine operates under such conditions, it can suffer what are known as "torque shocks." Such an ignition timing control system is known from, for instance, Japanese Patent Publication No. 61-6,262, entitled "Engine Control System," published on Feb. 25, 1986.
In the ignition timing control system described in the publication mentioned above, after interrupting fuel injection, effected for fuel economy and to reduce harmful emissions during deceleration, when the engine resumes fuel injection in order to change its operating condition, fuel ignition is made at a time which is delayed relative to a time corresponding to an ordinary ignition timing advance set according to engine operating conditions. After resuming fuel injection, the retarded ignition timing is gradually varied so that it approaches the timing corresponding to an ordinary ignition timing advance. Controlling the ignition timing in such a way suppresses a rapid increase in engine output torque when fuel injection is resumed, after it has been interrupted, so that the engine causes less of a torque shock.
The ignition timing retarding control is performed both when resuming fuel injection after it has been interrupted and when the engine output torque changes rapidly, such as when an automatic transmission shifts and causes a torque shock. In such an ignition timing retarding control, the fuel is ignited at a timing corresponding to an eventual ignition timing advance, which is determined based on a basic ignition timing advance, a correction advance of ignition timing, which is predetermined according to engine operating conditions, and a corrective retardation of ignition timing, which drops engine output torque.
In an engine having an ignition system which has, e.g., a rotary distributor, the eventual ignition advance is limited to within upper and lower limits. Such limitations are provided to ensure that a high spark plug voltage is available to provide sparks in a correct firing order. The eventual ignition timing advance range is predetermined so as to be narrow at lower intake air charging levels. Such corresponds to a higher intake air charging level range, which requires strict conditions to ensure a high voltage is applied from the distributor at the spark plugs to fire them properly. Consequently, even when performing the ignition timing retarding control to prevent the engine from possibly causing torque shocks due to a rapid increase in output torque, the eventual ignition timing advance must be kept above the lower limit so that a small difference is present between the eventual ignition timing advance and an ordinary ignition timing advance; such is equivalent to the basic ignition timing advance as corrected by the corrective advance of ignition timing. That is, the eventual ignition timing advance is corrected to delay the ignition timing in the retarding ignition timing control in order to suppress torque shocks with certainty. Even though this is true, the eventual ignition timing advance, which is determined based on (1) the basic ignition timing advance, (2) a corrective ignition timing advance and (3) a corrective retardation of ignition timing, is restricted by the lower limit and is actually corrected by an ignition timing retardation smaller than the corrective retardation of ignition timing. Consequently, when, for instance, the engine operates at lower intake air charging levels, the retarding control of ignition timing, in which the eventual ignition timing advance is set smaller than the ordinary ignition timing advance, develops an insufficient drop in engine output torque. Hence, torque shock is not effectively suppressed.